This invention relates generally to isotactic propylene homopolymer compositions and to methods for their production and use.
Multiple stage polymerization processes are known in the art as is the use of metallocene catalyst systems. Multiple stage polymerization processes, such as two-stage polymerization processes, are generally used to prepare block copolymers which contain rubbery materials. Two-stage polymerization process products may include propylene block copolymers. In some instances, the propylene/ethylene copolymer portion of these block copolymers may be rubbery. In these instances, these products may be more suitable for molding applications rather than films. In other instances, two or more metallocenes may be used for the preparation of isotactic propylene polymers.
Related patents and patent applications include: U.S. Pat. Nos. 5,280,074; 5,322,902; 5,346,925; 5,350,817; 5,483,002; 5,741,563; 6,063,482 and Canadian Patent Application No. 2,133,181
It has been discovered that isotactic olefinic polymer compositions, desirably propylene homopolymer compositions, may be made by polymerizing olefins such as propylene in one stage using a metallocene catalyst system and then further polymerized in a separate stage using the same catalyst system to further polymerize the polymer but to a different molecular weight. The different molecular weights are produced by varying the concentration of a chain transfer agent such as hydrogen.
The resulting polymers have surprisingly high molecular weight and broad molecular weight distribution, and offer processability benefits in many applications but particularly in oriented film applications. By xe2x80x9cbroad molecular weight distributionxe2x80x9d, it is meant that the ratio of the weight average molecular weight to number average molecular weight (Mw/Mn, or MWD) for the resulting polymer compositions of the present invention is from 2.5 to 20 in one embodiment, and greater than 3.5 in another embodiment, and greater than 4.0 in another embodiment, and greater than 4.2 in another embodiment, and greater than 4.5 in yet another embodiment, and greater than 4.8 in yet another embodiment, and greater than 5.0 in yet another embodiment, and less than 20 in yet another embodiment, and less than 10.0 in another embodiment, and less than 8.0 in yet another embodiment, wherein a desirable MWD range may be any combination of any upper limit with any lower limit described herein. Films made from these unique polymer compositions have a significantly broader processability range and can be evenly stretched at lower temperatures compared to the polypropylene films available today. The resulting films have a favorable balance of properties including high strength, good optical properties, excellent shrinkage and good barrier properties.
As such, this invention relates to a propylene polymer composition in one embodiment which includes propylene homopolymer, the composition possessing a molecular weight distribution in the range from 4.0 to 20.0 in one embodiment, and having hexane extractables of less than 2.0 wt % in another embodiment. When composition is formed into a film, the biaxially oriented film properties further characterize this propylene polymer composition. For example, the propylene polymer film, having pre-stretched dimensions of 50.8 mmxc3x9750.8 mmxc3x9720 mil (0.508 mm), exhibits an even stretch when stretched to a final stretched thickness of 0.75 mil (19 xcexcm) between the temperature ranges of from 149xc2x0 C. to 159xc2x0 C. on a T. M. Long biaxial stretching apparatus in one embodiment, and from 150xc2x0 C. to 158xc2x0 C. in another embodiment, and from 151.7xc2x0 C. to 157.2xc2x0 C. in yet another embodiment. Before stretching, the film is preheating for 27 seconds at the stretching temperature. The film is stretched at a rate of 76.2 mm/sec.
In another embodiment, the propylene polymer composition may include a blend of first and second propylene homopolymers. The first propylene homopolymer may have a melt flow rate in the range of 0.15 dg/min to 4.0 dg/min and a molecular weight distribution in the range of 1.8 to 2.5. The second propylene homopolymer may have a melt flow rate in the range of 5 dg/min to 1000 dg/min and a molecular weight distribution in the range of 1.8 to 2.5. The polymer composition thus has a broad MWD, such as from 4.0 to 20 in one embodiment.